Switching amplifiers provide far greater efficiency than their analog counterparts, primarily because transistors used to switch voltages to a load are either turned ON, so that the voltage across the transistor is relatively low, or turned OFF, so that the current through the transistor is relatively low. With either a low voltage across the transistor or a low current through the transistor, the power dissipated by the transistor is relatively low.
Although conventional switching amplifiers are widely used, they can produce excessive distortion in their output signals because of capacitive coupling between the gates of respective switching transistors used by such amplifiers. Another limitation of conventional switching amplifiers is that they can sometimes generate excessive electromagnetic radio frequency (“RF”) interference, that can interfere with the operation of the amplifier as well as with other electronic devices in the vicinity of the amplifier. This RF interference can be attenuated to some extent by coupling the load driven by the amplifier to filters formed by inductors and/or capacitors. However, the remaining RF interference can still be too high in some applications.
Attempts have been made to minimize the distortion of signals output from switching amplifiers by operating them in a balanced manner so that capacitive coupling to one side of a load is matched by capacitive coupling to the other side of the load. While this approach is successful in minimizing signal distortion, it actually increases the RF interference generated by the amplifier since the number of transistors switching must be increased.
Attempts have been made to minimize the frequency at which the peak amplitude of the RF interference occurs by varying to “dithering” the switching times of the transistors, but doing so tends to increase the distortion of the output signal since a signal input to the amplifier is not sampled at regular intervals. In addition, spread spectrum EMI reduction is limited by audible distortion products at higher deviations. This distortion is normally greater at higher output amplitudes, so it clearly is a distortion since it increases with amplitude.
There is therefore a need for a system and method for operating switching amplifiers in a manner that minimizes the magnitude of RF interference generated by the amplifier, and does so without introducing significant distortion.